In general a steel plate or sheet is coated with a paint and then supplied as a product for use as a body of, for example, automobiles, an outer plate or sheet of domestic electric appliances. In recent years, not only the quality of products but also the appearance of products, which appeals to the sense of sight has become regarded as being of importance. With respect to factors having an effect on appearance, it is said that appearance is greatly influenced by the surface roughness of the steel plate or sheet, although it depends also upon the thickness of the coating.
The roughness on the surface of the steel plate or sheet is formed by transferring unevenness of the surface of the roll to the surface of the steel plate or sheet at the time of cold rolling or skin pass rolling. In order to improve this fact, Japanese Examined Patent Publication (Kokoku) No. 62-11922, for example, discloses a method for processing a roll which comprises coating the surface of a roll with an ultraviolet curing photosensitive resin to form regular marks. In this method, however, the sensitivity of the photosensitive resin is so low that an energy of several tens to several hundreds of mJ/cm.sup.2 is necessary for curing the photosensitive resin. For this reason, when the photosensitive resin is cured by using an ultraviolet laser having an output of 100 W or more as an energy radiation, an exposure time of 50 msec is necessary, for example, in the case of a photosensitive film having a thickness of 7 .mu.m, and it takes a lot of time for a fine uneven pattern to be formed on the surface of the roll, which leads to a problem of remarkably poor productivity.
Further, in the conventional ultraviolet laser device, since it is difficult to continuously ensure a stable energy at a high output for a long period of time, the use of a high performance ultraviolet laser device requiring a high equipment cost becomes necessary for continuously ensuring a stable energy at a high output for a long period of time.